Injection and transfer molding of plastic are widely used techniques for encapsulation of electronic devices, particularly semiconductor devices. Typically, multiple semiconductor devices are mounted on a ladder lead frame. A two part mold is generally used. Each half of the mold has a stiff backplate which is mounted on a platen of a hydraulic press. The mating surfaces of the mold are called the parting line or parting surface.
Initially the two halves of the mold are held apart. One or more lead frames containing semiconductor devices are placed in an exposed half of the mold. The hydraulic press is then actuated and the mold closed at the parting surfaces, forming a cavity around each semiconductor die. It is not unusual for a mold to contain 10.sup.2 to 10.sup.3 cavities. Each of these cavities is connected by one or more gates, channels, and runners to one or more central reservoirs or pots in which a softened plastic is placed. A worm screw or ram compresses the plastic so that it flows into the cavities. As soon as the plastic has hardened, the mold is opened and the encapsulated assemblies removed.
Frequently, moveable pins are built into the mold to align the mold parts, to hold the lead frames in a particular location during molding or to provide automatic ejection of the encapsulated parts. Also, the mold may contain other moving parts such as variable gates, vents, and dams. Thus, molds for encapsulating electronic parts, particularly semiconductor parts, are often very complicated and have numerous mechanisms protruding from the backplates of the mold. The mold halves generally have stiff backplates which are spaced away from the press platens by heavy steel mounting blocks or columns so as to allow room for pin actuators or other mechanisms.
In order to push liquified plastic from the reservoirs into the many cavities, it is frequently necessary to inject or transfer the plastic at pressures exceeding 10.sup.3 Psi (6.9.times.10.sup.6 Pa). If the mold halves fail to seal tightly against each other or against the lead frames, undesired or unintended crevices may be present therebetween. At such high pressures, the crevices fill with plastic during encapsulation, producing thin webs of plastic where none are desired. These thin webs of plastic are referred to as flash and result generally from imperfect sealing of the mold. Before the encapsulated electronic devices can be used, this flash must be removed. This increases the cost of manufacture and is undesirable. Also, flash is a significant cause of mold wear, requires additional labor for mold cleaning between molding cycles, and increases mold down-time.
In order to minimize flash, great pains are generally taken to machine the mating surfaces of the mold halves flat and parallel where they are to seal. Usually, they are carefully inspected for planarity during manufacture and after installation in the press. Powerful hydraulic cylinders in the mold press are used to force the mold halves tightly against the lead frames and each other. However, the force which can be applied in an effort to seal the mold is limited, since excessive force causes coining of the lead frames and rapid mold wear. Despite these efforts, flash continues to occur, even in the most carefully fabricated molds. Often this flash is localized to particular portions of the mold.
In the prior art it has been common to attempt to eliminate flash by placing shims under some of the mounting blocks between the mold backplates and the press platen. While this may improve sealing in one region of the mold, it frequently causes flash to appear in another region of the mold. The larger the mold, the more severe the problem. The placement of shims varies from press to press and is accomplished by trial and error.
Another approach to flash elimination has been to use spring loaded moving inserts in the critical portions of the mold form itself where sealing must be achieved. The purpose of the moving inserts is to permit portions of the sealing surfaces in different parts of the mold to adjust or compensate for distortions in the mold, mounting blocks, or press. A disadvantage of this approach is that providing moving inserts makes the mold much more expensive to fabricate and operate. In particular, the moving inserts which are exposed to the plastic during transfer tend to bind and jam so that mobility is lost after only a short time in service. The mold must then be disassembled and cleaned to restore motion to the inserts. This is expensive and causes substantial mold down-time.
Thus, a need continues to exist for improved means and methods of encapsulation for electronic devices which eliminates unwanted flash.
Accordingly, it is an object of the present invention to provide an improved means for setting up molds for plastic encapsulation of electronic devices wherein the mating surfaces of the mold are arranged to seal uniformly across the mold so that flash is substantially eliminated.
It is a further object of the present invention to provide an improved means for mounting molds in molding presses so as to provide more perfect mold sealing to substantially eliminate flash.
It is an additional object of the present invention to provide an improved method for encapsulation of electronic devices wherein the occurrence of mold flash is substantially reduced.
As used herein in describing mounting of molds or mold backplates to press platens, the words mounting block or blocks are intended to include any attachment means and not be limited to a particular mounting block shape. For example, mounting blocks may be solid or hollow, and have a cross-sectional geometry which is square, round, rectangular, H-shaped, L-shaped, Z-shaped, or combinations thereof. Further, the mounting blocks may vary in cross-section along their length.